1. Field of the Invention
This invention relates to an antistatic polymer monofilament, a method for making an antistatic polymer monofilament for the production of spiral fabrics and spiral fabrics employing such monofilaments.
2. Description of Related Art
Industrial fabrics used as non-woven lay down conveyer belts in machines for making non-woven fabrics and paper machine clothing employed at the final dryer sections of paper making machines require antistatic properties. That is, they must dissipate or prevent the build up electrical charges to prevent such charges from accumulating to a point where high energy sparking may occur when the belts or clothing contact grounded objects. Such high energy sparking can create an undesired safety hazard. Permanent antistatic monofilaments are frequently included in industrial fabrics to prevent the undesired build up of static charges.
Current antistatic fabrics are very similar to conventional polyester dryer fabrics with the exception that they contain carbon filled polyamide antistatic yarns and conductive edging. The seams are either loop seams or endless. There is a disadvantage to both seam styles. The loop seam is weaker than the main body of the fabric and also can put an undesired seam mark onto the finished product. An endless seamed fabric is very time consuming to install, which also is undesirable.
British Patent GB 21 01 559 discloses a fabric that has metal wires woven together with polymer monofilaments, where the metal wires are capable of discharging electrostatic charges from the fabric. The disadvantage of this structure is that the stretching behavior of the metal wire differs significantly from that of the remaining fabric. This can easily lead to breakage of the metal wires. An additional risk is that the metal wires normally corrode in air, which creates points of interruption for discharging the static charges.
To overcome the above-identified problems of utilizing metal wires in conjunction with polymer monofilaments in the fabric, fabrics have been made with surface coated monofilaments, with the surface coating including conductive particles to dissipate undesired static charges. Thus, the conductive coating on the polymeric monofilaments renders the monofilaments antistatic without losing the benefit of monofilament mechanical properties. Examples of such coating treatments are disclosed in EP 0327227A2 (1989) to Xerox, EP 0308234 A 1(1989) to Courtaulds, EP 0294504 B 1(1987) to BASF, and U.S. Pat. No. 5,935,706 (1999) to Dupont. In the prior art coating processes a polymer monofilament typically is first treated to make it receptive for receiving the conductive coating. Thereafter, the conductive coating, e.g., carbon black or a layer of metal particles, are then suffused onto the treated surface of the polymer monofilament. Though this approach maintains most of the polymer monofilament properties, the antistatic sheath or coating tends to wear away, due to the fact that the coated monofilaments frequently are subject to strong mechanical abrasion. In particular, during the fabric weaving process, friction between the coated monofilaments and mechanical parts of the weaving machine tend to abrade the coated sheath and cause serious housekeeping problems. Moreover, the monofilament treatment and coating process involves complicated chemical processing, thereby undesirably increasing the cost of the formed fabric.
Another prior art practice has been to incorporate antistatic particles into polymer bodies instead of employing a surface coating. A variety of materials, including carbon fibers, metallic fibers and carbon blacks can be used as polymer additives for making antistatic or conductive polymer compositions. Such approaches are disclosed in, e.g., U.S. Pat. No. 6,083,562 (2000) to Sterling Chemicals International and EP 0399397 A2 (1990) to Dupont. The desired electrical conductivity in the polymer composite is achieved when an adequate network of conductive particles or fibers is established within the polymer. Large amounts of carbon black or metal particles, typically in an amount >10 wt %, need to be incorporated into the polymer to form the desired network for dissipating static charges.
The above-described polymer composites including conductive additives have not been reported for use in polymer monofilaments. This is largely due to the fact that the composite does not have good compatibility between the polymer matrix and the highly loaded, large particle-sized antistatic fillers. In particular, carbon black, carbon fibrils and metal particles, typically in a dimension of microns, lead to severe phase separation during formation (i.e., extrusion and orientation in the solid state) of monofilaments, resulting in very brittle filaments having unacceptable variations in diameter along their length. Such monofilaments would be unsuitable for use in applications of this invention, which require the monofilaments to have a high degree of structural integrity to maintain their mechanical properties and close dimensional tolerances along their length.
Graphite nanotubes as a conductive medium are a relatively new entry in this general area. Compared to carbon blacks, which typically have a particle size on the order of several microns, the multiple wall nanotubes have a diameter between 10–20 nm and an aspect ratio of greater than 100, making them sub-micron in size. The surface area of these nanotubes typically can be about 250 m2/g and can establish a conductive network in a polymer matrix at very low loadings, resulting in minimal degradation of polymer physical properties. EP 1054036 A1 (2000) to Fina Research S.A. discloses partially oriented fibers made of polyethylene and polypropylene with carbon nanotubes. WO 02/076724 (2002) to Eikos disclosed electrically conductive polymer films containing carbon nanotubes. These disclosures, however, are not readily applicable to, nor do they suggest forming monofilaments.
Polymer monofilaments, which are the subject of the instant invention, differ from fibers and films of the type disclosed in EP 1054036 and WO 02/076724 in that the primary purpose of the carbon nanotubes in the present invention is to provide a network capable of dissipating a static charge, and that the diameter of a round cross sectional area of the formed monofilament is greater than 0.05 mm. Any fillers in the polymer matrix can easily cause phase separation in the monofilament orientation process as the monofilament exits the extruder.
It is an object of this invention to develop polymer monofilaments that exhibit permanent static dissipative properties with long lasting effect and good mechanical properties.
All references cited herein are incorporated herein by reference in their entireties.